AT&T kicked off Wi-Fi calling on newer-model iPhones running iOS 9 on Thursday after winning permission from the Federal Communications Commission two days earlier.

Wi-Fi Calling helps users get better connections indoors where cellular service can be spotty. Where permitted by a carrier, it can also eliminate international calling costs of up to $1 a minute.

T-Mobile and Sprint already offer Wi-Fi calls on certain devices, and T-Mobile started the practice as early as 2007 without securing the same permission from the FCC that AT&T received.

AT&T said the iPhone 6S, iPhone 6S Plus, iPhone 6 and iPhone 6 Plus will support Wi-Fi Calling if they have iOS 9 installed.

To add Wi-Fi calling to an eligible iPhone, according to Apple's website, go to Settings> Phone> Wi-Fi Calling. You will then be prompted to answer a few questions.

With AT&T's announcement, Verizon Wireless is expected to follow suit.

T-Mobile allows customers on certain devices to make Wi-Fi calls "virtually anywhere" there is Wi-Fi access. However, AT&T said its Wi-Fi Calling service will be available only when calling or texting from the U.S., Puerto Rico and the U.S. Virgin Islands.

AT&T didn't offer an explanation for its restriction to those geographies.

In a blog posted Tuesday, AT&T bemoaned that T-Mobile and Sprint were allowed to move ahead so much earlier, without receiving the same permission in the form of a waiver that AT&T sought and received.

"We are left scratching our heads as to why the FCC still seems intent on excusing the behavior of T-Mobile and Sprint who have been offering these services without a waiver for quite some time," said Jim Cicconi, senior executive vice president of external affairs at AT&T.

The FCC waiver permits AT&T to begin offering Wi-Fi calling without also offering teletypewriter (TTY) communications for the deaf, hard of hearing and speech-impaired.

AT&T wants to set up RTT (Real Time Texting) instead, arguing it works better over the Internet. Once implemented, RTT would be backward compatible with TTY, AT&T said in a blog in July.

Roger Entner, an analyst at Recon Analytics, said AT&T probably sought and received the waiver to avoid an FCC fine for proceeding without permission.

He said Verizon has also proceeded slowly on Wi-Fi calling, hoping also to avoid a fine for the same reason.

"Verizon has not launched Wi-Fi calling but now that AT&T has the waiver, I would expect Verizon to launch shortly," Entner said. "Sprint and T-Mobile didn't bother to get a waiver and apparently they are less afraid of the FCC. Historically, they have gotten nicer treatment from the FCC."

AT&T and Verizon are the nation's largest carriers "and are trying not to give the FCC an excuse to fine them for something," Entner said. Entner said Verizon especially has traditionally been hyper-attentive to legal details. "You know, people say that Verizon is a law firm with a lot of wireless antennas on top," he said.

Entner said there is not any technology reason for AT&T to restrict Wi-Fi calling to the U.S., Puerto Rico and the Virgin Islands. For business travelers trying to lower costs when abroad, calls made on Wi-Fi can sidestep international calling rates of 20 cents to $1 a minute, he said.

"Again, maybe AT&T is being careful on [geographic coverage] for a legal reason," he said.